1. Field of the Invention
The present invention relates to a circuit substrate provided with a transistor for use in an active matrix liquid crystal display apparatus or other electronic circuits.
2. Description of the Related Art
A transistor device formed on a circuit substrate, such as an active matrix substrate or the like, includes a gate electrode, a source electrode and a drain electrode. In a semiconductor fabrication process, a silicon material excellent in heat resistance and a metal material having a high melting point are conventionally used for forming the gate electrode, because these materials are resistant against high temperature.
However, such an electrode material raises the resistance of wirings when used in forming the wirings for a liquid crystal display apparatus or the like. Consequently, materials having low resistance have been examined for their applicability. Recently, a technology for forming a transistor device or the like using a low temperature technique has been developed, leading to the use of a low resistance metal material such as aluminum for forming the gate electrode.
In the case where a low-resistance metal material, such as aluminum, is employed for forming electrodes, the advantageous feature that electric insulation is ensured when an anodization film is formed on the surface of the electrodes is realized. This is because the anodization film provides corrosion-resistance for the electrodes and prevents the deterioration of the electrodes due to hillocks occurring in the electrodes.
In addition, when such a low-resistance metal is used for forming the gate electrode of a transistor for controlling a pixel to be in an ON state or OFF state in a liquid crystal display apparatus and a gate insulation film is formed by anodizing the surface of the gate electrode, offset regions can be formed in a source region and a drain region of a semiconductor layer of the transistor, using the anodization film. More specifically, the source and drain regions are formed in the semiconductor layer by implanting ions into the semiconductor layer to sandwich a channel region located below the gate electrode. Due to the existence of the anodization film formed on the gate electrode, spaces having a width in a channel length direction of the transistor which is equal to the thickness of the anodization film are formed between the edges of the gate electrode and the edges of the source region and the drain region. This space is the offset region. With such a structure, an amount of current leaked between the gate electrode, the source region and the drain region in the transistor can be reduced. Also, the withstand voltage of the drain is improved. As a result, a reliable liquid crystal display apparatus can be obtained.
As described above, in the case of forming the anodization film on the surface of the electrode by an anodization method, various advantages can be provided. However, some problems arise in the anodization process. For example, anodization of metal electrodes is conducted by making current flow through the metal electrodes while a substrate provided with the metal electrodes thereon is immersed in a solution. Therefore, in order to obtain the anodization film having the desired characteristics, it is necessary to carefully control the voltage, current and the period of time during the current flow. In addition, the thickness of the anodization film is different in every production, due to deterioration of the solution or the like.
Furthermore, there is another problem in the case of a driving circuit-integrated liquid crystal display apparatus where a pixel transistor and a driving transistor for driving the pixel transistor are both formed on the same substrate. The driving transistor requires a large ON current in order to increase an operation frequency. On the other hand, the pixel transistor requires a small OFF current in order to prevent image signals from leaking. To achieve this, it is preferable that the offset width in the channel length direction of the driving transistor be short, whereas that of the pixel transistor be long. Accordingly, several types of offset regions having different widths in the channel length direction are required in one substrate depending on the purpose of use of the transistors, and therefore several types of anodization films having different thicknesses are required. However, in order to form the anodization films having different thicknesses, it is necessary to vary the voltage and current during anodization depending on the desired thickness by using several voltage terminals. Alternatively, a period of time may be changed to vary the thicknesses of the anodization films. However, this complicates the production process of the liquid crystal display apparatus having a substrate on which a pixel transistor and a driving circuit including driving transistors are formed.